GCN Circular 8547
B. E. Cobb (UC Berkeley) reports: Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained several epochs of optical/IR imaging of the error region of GRB 081121 (GCN 8537, Oates et al.). Each epoch consisted of several dithered images in each filter, with total summed exposure times of 180s in each of BRIJK and 120s in each of H and V. At a mid-exposure time of 2008-11-22 02:02 (5.4 hrs post-burst), the GRB afterglow (GCN 8536, Yuan et al.) is detected with the following magnitudes: B = 19.34 +/- 0.04 R = 18.61 +/- 0.04 I = 18.21 +/- 0.04 J = 17.21 +/- 0.15 H = 17.05 +/- 0.15 K = 16.75 +/- 0.15 Observations were obtained under non-photometric conditions. In optical, these preliminary magnitudes are calibrated against several USNO-B1.0 stars in the field, so there is likely an additional photometric calibration error of ~0.3 magnitudes. In the IR, calibration is against 2MASS stars. Between 5.4 hrs and 8.7 hours post-burst, the afterglow decays with an approximate optical decay rate of alpha~-1 (where afterglow flux is proportional to t^alpha).